Thermoplastic injection molding (TIM) is a well known and widely used technique to manufacture a large variety of plastic or polymeric products. In general, TIM involves forcing molten thermoplastic materials into a mold wherein, upon cooling, they form a solid polymeric article in the shape defined by the mold. In such processes, the thermoplastic material is typically melted in a heated chamber or barrel of an injection molding machine, whereupon it is injected into the mold. While TIM has enjoyed wide use in the manufacture of plastic articles, it has rarely been used in the manufacture of asphalt-based products.
With the recent development of asphalt-based compositions suitable for the manufacture of shaped articles, a need has arisen for molding processes and equipment capable of mass producing such articles economically and efficiently. Although these asphalt-based compositions can be formed into articles by molding techniques such as vacuum molding or compression molding, a suitable injection molding process would, in most high volume circumstances, be preferable because of its high speed and low cost in making products and its wide acceptance and usage in the parts manufacturing industry. One of the main virtues of an asphalt-based composition is its low material cost. However, compounding asphalt with thermoplastics negates a large part of that cost advantage. Accordingly, a need exists for an injection molding process and apparatus suitable for the injection molding of compositions containing asphalt and thermoplastic polymers that mix the thermoplastic polymers and asphalt directly in the injection molding machine. These needs are met by the injection molding process and apparatus described herein.